The present invention relates to reticle (mask) holders (i.e., xe2x80x9cchucksxe2x80x9d) as used in microlithography apparatus and methods employed in the manufacture of semiconductor devices, displays, and the like. More particularly, the invention pertains to reticle chucks useful in a low-pressure atmosphere as encountered in charged-particle-beam microlithography.
Microlithography is a key technology used in the fabrication of semiconductor integrated circuits, displays, and the like. In microlithography, an image of a circuit pattern, defined by a mask or reticle, is projected onto the surface of a xe2x80x9csensitizedxe2x80x9d substrate such as a semiconductor wafer coated with a suitable xe2x80x9cresist.xe2x80x9d
In view of the resolution limits of optical microlithography, a large amount of effort currently is being devoted to the development of microlithography systems that use a charged particle beam (e.g., electron beam or ion beam) to transfer a pattern, defined on a mask or reticle, to a sensitized substrate (e.g., semiconductor wafer). Charged-particle-beam (CPB) microlithography offers prospects of improved resolution compared to optical microlithography.
FIG. 3 shows a CPB microlithography system for projection-exposing a reticle pattern. An electron beam EB emitted from an electron gun 31 propagates along an axis AX and is collimated by a condenser lens 32. The electron beam is deflected within an XY plane by a field-selection deflector 33 to direct the beam to a xe2x80x9csubfieldxe2x80x9d or other exposure unit of a reticle 50. A xe2x80x9csubfieldxe2x80x9d is a region of the reticle 50 hat is illuminated by the electron beam at any given instant of time, and normally defines a small respective portion of the overall pattern defined by the reticle. The electron beam propagating from the electron gun 31 to the reticle 50 is termed the xe2x80x9cillumination beam.xe2x80x9d As the illumination beam passes through the reticle 50, the beam acquires an ability to form an image of the illuminated subfield, and hence is termed a xe2x80x9cpatterned beam.xe2x80x9d The patterned beam experiences a prescribed magnitude and direction of lateral deflection imparted to the beam by a deflector 34 that causes the patterned beam to be incident on a prescribed region of the substrate (xe2x80x9cwaferxe2x80x9d) 9. The illuminated region of the wafer 9 corresponds to the particular subfield of the reticle 50 actually being illuminated by the illumination beam. As the patterned beam propagates to the wafer 9, the patterned beam passes through first and second projection lenses 35, 36, respectively, (collectively comprising a xe2x80x9cprojection-lens systemxe2x80x9d) to form an image of the respective pattern portion on the surface of the wafer 9. The size of the image as formed on the wafer 9 is xe2x80x9creducedxe2x80x9d (demagnified) according to a prescribed demagnification ratio of the projection-lens system.
The reticle 50 is mounted on a reticle stage 37 so as to extend parallel to an X-Y plane (in FIG. 3, the X-axis extends perpendicularly to the plane of the page). During microlithographic exposure of the wafer 9, the reticle stage 37 is driven continuously in the X-axis direction and stepwise in the Y-axis direction by a stage driver 38. The position of the reticle stage 37 in the X-Y plane is sensed by a respective laser interferometer 39 that produces respective output signals routed to a controller 24.
The wafer 9 is mounted on a wafer stage 12 and extends in an X-Y plane parallel to the reticle 50. During exposure, the wafer stage 12 is driven continuously in the X-axis direction and stepwise in the Y-axis direction by a stage driver 40. Because the image is inverted by the projection lenses 35, 36, the direction of travel of the wafer stage 12 in both the continuous-motion direction and the stepwise-motion direction during exposure are opposite the corresponding motions of the reticle stage 37. The position in the X-Y plane of the wafer stage 12 is sensed by a respective laser interferometer 41 that produces output signals routed to controller 24.
Deflector power supplies 42, 43 provide electrical power to the deflectors 33, 34, respectively, under control of the controller 24.
FIG. 4 illustrates various relationships extant between the reticle 50 and the wafer 9. The areas 60 shown on the wafer 9 are xe2x80x9cdies.xe2x80x9d (A die is a separate area on the wafer 9 into which an entire pattern from the reticle 50 is to be transferred.) For use in a microlithography apparatus such as that shown in FIG. 3 (in which projection-exposure is performed by dividing the reticle pattern into subfields sized for the particular optical field of the projection-lens system), the pattern 51 of the reticle 50 is divided into multiple regions 51a, 51b, 51c termed xe2x80x9cstripes.xe2x80x9d Each stripe has a length dimension that extends in the direction (X-direction) of continuous motion of the reticle 50. Each stripe 51a, 51b, 51c is further divided into multiple subfields 52 arranged by rows in each stripe. In a similar manner, each of the dies 60 on the wafer 9 can be represented as multiple xe2x80x9cstripesxe2x80x9d 61a, 61b, 61c, wherein each stripe is further divided into multiple xe2x80x9ctransfer subfieldsxe2x80x9d 62.
During exposure of a pattern image onto a die 60, as the reticle 50 and wafer 9 complete one cycle of continuous motion, the respective pattern portions contained in each of the subfields 52 of a stripe 51a, 51b, 51c are projection-exposed, one after the other, into corresponding transfer subfields 62 of a respective stripe 61a, 61b, 61c. For example, in FIG. 4, as the reticle 50 moves continuously in the xe2x88x92X-direction (arrow B1) and the wafer 9 moves continuously in the +X-direction (arrow C1), the electron beam EB is deflected back and forth along the Y-axis (arrow D) to thus scan, in sequence, each of the subfields 52 of the stripe 51a. After completing exposure of the stripe 51a in this manner, the reticle 50 is stepped in the xe2x88x92Y-direction (arrow B2) and the wafer 9 is stepped in the +Y-direction (arrow C2). Next, as the reticle 50 and the wafer 9 are moved continuously in the +X- and xe2x88x92X-directions (arrows B3 and C3), respectively, the electron beam EB is deflected so as to illuminate, in sequence, the subfields 52 of the stripe 51b. After completing exposure of the stripe 51b, the reticle 50 is stepped in the xe2x88x92Y-direction (arrow B4) and the wafer 9 is stepped in the +Y-direction (arrow C4). Then the reticle 50 and wafer 9 are moved continuously in the xe2x88x92X- and +X-directions (arrows B5 and C5), respectively, to expose the stripe 51c. This sequence of exposure steps is repeated until the reticle pattern has been exposed in each of the dies 60 of the wafer 9.
FIG. 5 shows a conventional reticle holder (xe2x80x9creticle chuckxe2x80x9d) 500 suitable for mounting to the reticle stage 37 (FIG. 3) and configured to hold a reticle substrate 101. The reticle substrate 101 is placed on the reticle chuck 500 which holds the reticle by electrostatic attraction. The reticle chuck 500 comprises electrodes 503, 503xe2x80x2 situated depthwise in the reticle chuck 500. The electrodes 503, 503xe2x80x2 are connected to an electrode controller 502. Whenever the electrode controller 102 applies a voltage to the electrodes 503, 503xe2x80x2, an electrostatic force is created that attracts and thus urges the reticle substrate 101 against the surface of the reticle chuck 500. In a region 504 of the reticle, as indicated in the figure, the reticle pattern can be defined, and an area 505 within the region 504 is where the pattern actually is defined.
An apparatus including an electrostatic chuck is disclosed, for example, in Japanese Kxc3x4kai Patent Document No. Hei 10-050584. Additional examples of electrostatic wafer chucks for use in other semiconductor manufacturing processes are disclosed in Japanese Kxc3x4kai Patent Document Nos. Hei 5-063062 and Hei 7-283297. For more information on electrostatic chuck operation, reference is made to Yoshihiro Kubota, xe2x80x9cSeiden Chakku to Sono Ouyou (xe2x80x9cThe Electrostatic Chuck and its Application,xe2x80x9d Denshi Zairyou (Electronics Materials), July 1996, page 51.
Methods for fabricating certain types of reticles for use in CPB microlithography systems include an etching step to xe2x80x9cmachinexe2x80x9d a silicon wafer in a manner yielding a reticle membrane having a thickness of 1-3 micrometers, and to form, in the membrane, openings (voids) shaped to define respective pattern features. Such a reticle is termed a xe2x80x9cscattering-stencil reticle.xe2x80x9d Methods for fabricating other types of reticles include a step to machine a silicon wafer in a manner yielding a reticle membrane having a thickness of 1 micrometer or less. The pattern is defined by a corresponding pattern of a scattering material formed on the surface of the membrane. Such a reticle is termed a xe2x80x9cscattering-membrane reticle.xe2x80x9d (See, for example, Japanese Kxc3x4kai Patent Document No. Hei 10-116782.)
Whenever microlithography is performed using a conventional electrostatic reticle chuck, movement of the reticle stage typically results in lateral shifting of the reticle relative to the reticle chuck. That is, whenever the reticle stage is moved (as described above) during exposure, the reticle is subjected to lateral forces that are highest at moments of greatest accelerations and decelerations of the reticle stage. Lateral reticle shifting occurs because the lateral forces generated by the accelerations and decelerations of the reticle stage are greater than the electrostatic attractive force produced by a conventional electrostatic chuck. This situation results in shifts and faulty registration of pattern features between adjacent layers, which greatly reduces production yields.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide reticle chucks and reticle-holding methods that securely hold a reticle during use. Another object is to provide semiconductor device-manufacturing methods in which excessive time is not required for reticle-positioning alignments, thereby providing improved manufacturing yields.
To such ends, and according to a first aspect of the invention, electrostatic reticle chucks are provided for holding a reticle as used in charged-particle-beam microlithography. A first embodiment of such a chuck comprises a reticle-contacting surface formed of a dielectric material, and at least one electrode situated beneath the reticle-contacting surface of the dielectric material. The dielectric material (at least along the reticle-contacting surface) has a volume resistivity of no greater than 1013 xcexa9-cm at ambient temperature. At such a volume resistivity, a strong electrostatic reticle-attractive force can be generated due to the Johnsen-Rahbek effect. In general, with Johnsen-Rahbek-type electrostatic attraction, the operations of reticle attraction and release can take more time than required when the attractive force is a Coulomb force. However, the attractive force is greater with the Johnsen-Rahbek effect. Fortunately, with a reticle chuck, the time required to develop or release the attractive force does not affect overall microlithographic throughput. Rather, the key benefit is the large and stable attractive force that can be obtained. Also important in producing the Johnsen-Rahbek effect is the reticle-contact surface(s) of the reticle chuck.
The reticle chuck summarized above is especially suitable for holding a reticle made from a silicon reticle substrate.
An electrostatic reticle chuck according to another embodiment comprises a reticle-contacting surface formed of a dielectric material, and at least one electrode situated beneath the reticle-contacting surface of the dielectric material. The reticle chuck is configured to attract and hold a reticle by Johnsen-Rahbek electrostatic attraction. Whereas the dielectric material beneath which an electrode is situated desirably has a volume resistivity of no greater than 1013 xcexa9-cm at ambient temperature, the dielectric material desirably has a volume resistivity of at least 108 xcexa9-cm at ambient temperature. This range of volume resistivity allows the reticle chuck to hold the reticle sufficiently strongly to accommodate reticle-stage accelerations and decelerations as high as 1 xc3x97g.
As discussed later below, the speed with which the reticle is secured to or removed from the reticle chuck is not a significant factor of microlithographic throughput. Hence, even though more time normally is required to turn a Johnsen-Rahbek force on and off, compared to a Coulomb force, such increased time is immaterial with respect to throughput because the time is expended in securing a reticle to or removing a reticle from the reticle chuck. Thus, the benefits of the Johnsen-Rahbek force can be exploited in this invention without any detriment.
Because the Johnsen-Rahbek force holds the reticle more strongly, per unit area of reticle actually contacting the surface of the reticle chuck, than a Coulomb force, use of the Johnsen-Rahbek force also allows the area of the reticle actually contacting the reticle chuck to be reduced. This allows more of the area of the reticle to be used for pattern definition. In addition or alternatively, the portion of the reticle actually contacting the reticle chuck can be spaced more distantly on the reticle from the patterned region, thereby reducing the probability of damage to the patterned region during use.
Any of the reticle chucks according to the invention can be configured to attract and hold a reticle mounted in a reticle frame, wherein the Johnsen-Rahbek electrostatic attraction acts on the reticle frame.
According to another aspect of the invention, charged-particle-beam microlithography apparatus are provided. An embodiment of such an apparatus comprises, in order along an axis, a charged-particle-beam source, an illumination-optical system, a reticle stage, a projection-lens system, and a wafer stage. The reticle stage is configured, for example, as any of the reticle stages summarized above.
According to yet another aspect of the invention, methods are provided for holding a reticle for use in charged-particle-beam microlithography. In an embodiment of such a method, a reticle-contacting surface is provided on an electrostatic chuck. The reticle-contacting surface comprises a dielectric material. At least one electrode is situated beneath the reticle-contacting surface. The dielectric material situated between the electrode and the reticle-contacting surface has a volume resistivity of no greater than 1013 xcexa9-cm (preferably greater than 108 xcexa9-cm) at ambient temperature. The electrode is energized so as to generate an electrostatic force that attracts and holds the reticle to the reticle-contacting surface. As a result of the stronger reticle-holding force provided by the reticle chuck, the position of the reticle on the chuck does not shift whenever the reticle stage moves. This eliminates the need to correct faulty reticle positioning, resulting in improved throughput.
In another embodiment of a method according to the invention, a reticle-contacting surface comprising a dielectric material is provided on an electrostatic chuck. At least one electrode is provided beneath the reticle-contacting surface. The dielectric material situated between the electrode has a dielectric property sufficient, when the electrode is energized, to attract and hold a reticle by Johnsen-Rahbek electrostatic attraction. The reticle is attracted to and held by the reticle-contacting surface whenever the electrode is energized so as to generate the Johnsen-Rahbek electrostatic force.
The invention also encompasses methods for performing CPB microlithography of a pattern, defined by a reticle, onto a wafer substrate. In an embodiment of such a method, a reticle is provided that defines the pattern. The reticle is held in a reticle chuck as summarized above. The reticle chuck is mounted on a reticle stage. An illumination beam is passed through the reticle to form a patterned beam. The patterned beam is passed through a projection-lens system to a sensitive substrate.
The invention also encompasses methods for manufacturing a semiconductor device, in which methods CPB microlithography is performed as summarized above.
The foregoing and other features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.